It's
also important to develop your code with Perl
reporting every possible relevant warning. Under mod_perl, you can
turn this mode on globally, just like you would by using the
-w command-line switch to Perl. Add this
directive to httpd.conf:

PerlWarn On

In Perl 5.6.0 and later, you can also enable warnings only for the
scope of a file, by adding:

use warnings;

at the top of your code. You can turn them off in the same way as
strict for certain blocks. See the
warnings manpage for more information.

We will talk extensively about warnings in many sections of the book.
Perl code written for mod_perl should run without generating any
warnings with both the strict and
warnings pragmas in effect (that is, with
usestrict and
PerlWarnOn or
usewarnings).

Warnings are almost always caused by errors in your code, but on some
occasions you may get warnings for totally legitimate code.
That's part of why they're warnings
and not errors. In the unlikely event that your code really does
reveal a spurious warning, it is possible to switch off the warning.